1. Field of the Invention
The present invention relates to a toner for developing electrostatic images, a method for producing the toner for developing electrostatic images, a developer for developing electrostatic images, an image forming method, an image forming apparatus, and a process cartridge using the toner for developing electrostatic images.
2. Description of the Related Art
In electrophotographic apparatuses, electrostatic recording apparatuses, or the like, a toner is made to adhere on a latent electrostatic image formed on a photoconductor, the toner is transferred onto a transferring material, and the toner is fixed onto the transferring material by means of heat to thereby form a toner image. In full-color image formation, a color image is typically reproduced using four-color toners of black, yellow, magenta, and cyan, the image is developed for each of the four-color toners, respective toner layers of the four-color toners superimposed on a transferring material are fixed at a time by heating to thereby obtain a full-color image.
From the standpoint of users who are generally familiar with printed materials, images obtained with a full-color copier are not of satisfactory level. Further higher quality image formation satisfying high-fineness and high-resolution levels which are close to those of photographs and printing is demanded. It is known that a toner having a small particle diameter and a narrow particle size distribution is used in high-quality image forming of electrophotographic images.
Conventionally, electronic or magnetic latent images are developed using a toner. A toner used for developing electrostatic images is colored particles in which a colorant, a charge controlling agent, and other additives are contained in a binder resin, and there are two main types of methods for producing such a toner, i.e. pulverization method and polymerization method. In pulverization method, a colorant, a charge controlling agent, an offset inhibitor, or the like are fused and mixed in a thermoplastic resin to be uniformly dispersed therein, the obtained composition is pulverized, and the pulverized toner particles are classified to thereby produce a toner. According to pulverization method, a toner having rather excellent properties can be produced, however, there are limitations on selection of materials for the toner. For example, a composition to be obtained by fusion and mixture of toner materials needs to be pulverized and classified through use of an economically available apparatus. Because of the needs, it leaves no alternative but to make a fused and mixed composition sufficiently brittle.
For the reason, when the composition is actually pulverized into particles, a wide range of particle size distribution is easily formed. When a copied image having high-resolution and high-tone is tried to be obtained, for example fine power particles having a particle diameter of 5 μm or less and coarse powder particles having a particle diameter of 20 μm or more must be removed in a classification process, and thus there is a disadvantage that the yield is extremely low. In addition, when a pulverization method is employed, it is difficult to uniformly disperse a colorant, and a charge controlling agent in a thermoplastic resin. Ununiform dispersion of compounding ingredients adversely affects the flowability, developing property, durability, image quality of the toner.
In recent years, in order to overcome the problems in these pulverization methods, for example, toner particles are obtained by suspension polymerization method (see Japanese Patent Application Laid-Open (JP-A) No. 09-43909). However, the toner particles obtained by suspension polymerization method are spherically shaped, and there is a disadvantage that the toner particles are poor in cleaning ability. In developing and transferring an image having a low image area ratio, the amount of residual toner particles after transferring is small, and thus there is no problem with cleaning ability, however, an image having a high image area ratio such as a photographic image, further, a toner with which an untransferred image is formed due to a sheet-feeding failure or the like may occur as a residual untransferred toner on a photoconductor, causing background smear of image when such a residual untransferred toner is accumulated.
In addition, it causes smears on charge rollers or the like which contact-charges the photoconductor, which disenables exerting of its intrinsic chargeability thereof.
On the other hand, a method for obtaining toner particles formed in indefinite shape by associating resin fine particles obtained by an emulsion polymerization method each other has been disclosed (see Japanese Patent (JP-B) No. 2537503). However, in the toner particles obtained by the emulsion polymerization method, a large amount of surfactants remains not only on the surface of the toner particles but also in the inside of the toner particles even when they have been subjected to a washing treatment, which causes impaired environmental stability of toner charge, a widen charge amount distribution, and image defective due to smears of the obtained images. There are problems that the remaining surfactants smear the photoconductor, charge rollers, developing rollers, or the like, which disenables exerting of its intrinsic chargeability.
On the other hand, in a fixing step according to a contact-heat method in which fixing is performed by means of heating members such as a heat roller, releasing property of toner particles against the heating members, which is hereinafter referred to as anti-offset property, is required. Anti-offset property can be improved by making a releasing agent reside on surfaces of toner particles. In view of this tendency, Japanese Patent Application Laid-Open (JP-A) No. 2000-292973 and Japanese Patent (JP-B) No. 3141783 respectively disclose a method in which anti-offset property is improved by making resin fine particles reside not only in toner particles but also are unevenly distributed onto surfaces of the toner particles. However, this method involves a problem that the lower limit fixing temperature is raised, causing insufficient low-temperature fixing property, i.e. energy-saving fixing property.
In the method in which resin fine particles obtained by emulsion polymerization method are associated each other to thereby obtain a toner formed in indefinite shape, the following problems are caused. In other words, in the case where fine particles of a releasing agent are associated with toner particles in order to improve anti-offset property, the fine particles of the releasing agent are substantially taken into the toner particles, resulting in discouraging improvement in anti-offset property with sufficiency. Since resin fine particles, fine particles of releasing agents, fine particles of colorants or the like are fused and bound to toner particles randomly to thereby form the toner particles, variations arise in the composition or ratio of contents of the components between the obtained toner particles, and in molecular mass of the resin or the like, resulting in different surface properties between the toner particles, and disenabling of forming images steadily over a long period of time. Further, in a low-temperature fixing system in which low-temperature fixing property is required, there has been a problem that fixing is inhibited due to resin fine particles which reside on surface of the toner, which disenables ensuring the range of fixing temperatures.
On the other hand, a new method of producing a toner called the Emulsion-Aggregation method (EA method) is recently disclosed (Japanese Patent (JP-B) No. 3141783). In this method, toner particles are granulated from polymers which have been dissolved in an organic solvent or the like, contrary to the suspension polymerization method in which toner particles are formed from monomers. Japanese Patent (JP-B) No. 3141783 discloses some advantages of the emulsion-aggregation method in terms of an expansion of selection range of resins, controllability of polarity, and the like. In addition, it is advantageous in capability of controlling a toner structure, i.e. controlling a core-shell structure of toner particles. However, the shell structure comprises a layer containing only resins and aims for reducing the amount of pigments and waxes exposed on surface of toner, and it is disclosed that the toner is not innovative in its surface condition and does not have an innovative structure (The 4th-Joint Symposium—the Imaging Society of Japan and the Japan Society of Static Electricity (held on Jul. 29, 2002)). Thus, a toner produced by the emulsion-aggregation method is formed in a shell-structure, however, the toner surface comprises generally used resins and does not have an innovative structure, and there is a problem that when further lower-temperature fixing is pursued, it is not sufficient in heat resistant storage stability, and environmental charge stability.
In addition, in any of the suspension polymerization method, the emulsion polymerization method, and the emulsion aggregation method, styrene-acrylic resins are typically used, and with the use of polyester resins, it is difficult to granulate toner and difficult to control particle diameter, particle size distribution, and shape of toner. When further lower-temperature fixing is pursued, there are limitations in fixing property.
Further, aiming for excellent heat resistant storage stability and low-temperature fixing, using a polyester modified with urea-bonding has been known (Japanese Patent Application Laid-Open (JP-A) No. 11-133667), however, the surface of the toner is not particularly contrived, and there is a problem in environmental charge stability under strict conditions.
In the field of electrophotography, obtaining high-quality of images has been studied from various angles. Among these studies, it has been increasingly recognized that making toner in smaller diameter and in a spherical form is extremely effective in obtaining high-quality of images. There seems to be tendencies that with increasingly smaller diameter of toner, transferring property and fixing property are lowered, which leads to poor images. It has been known that transferring property is improved by forming a toner in a spherical shape (Japanese Patent Application Laid-Open (JP-A) No. 09-258474).
In these circumstances, in the fields of color copiers and color printers, further higher-speed image forming is required. To respond to higher-speed image forming, an apparatus employing tandem-type technique is effectively used (Japanese Patent Application Laid-Open (JP-A) No. 05-341617). The tandem-type technique is a technique by which images formed by an image forming unit are sequentially superimposed and transferred onto a single transferring paper sheet transported by a transferring belt to thereby obtain a full-color image on the transferring paper sheet. A color image forming apparatus based on the tandem-type technique has excellent characteristics of allowing a variety types of transferring paper sheet for use, having high-quality of full-color image, and enabling full-color images at high speeds. In particular, a capability of obtaining full-color images at high speeds is a characteristic unique to the tandem-type technique. The characteristic is not found in a color image forming apparatus employing other techniques.
On the other hand, there have been attempts to achieve high-quality image as well as speeding-up using a toner formed in a spherical shape. To respond to further higher-speeding up, speedy fixing property is required, however, a spherically-shaped toner satisfying excellent fixing property as well as excellent low-temperature fixing property has not yet been realized so far.
In addition, when a toner is stored and delivered after production of the toner high-temperature and high humidity environment, low-temperature and low humidity environment are harsh conditions for the toner. A toner of which toner particles do not flocculate each other during the time of storage, has no degradation or exhibits less degradation in charge property, flowability, transferring property, and fixing property, and excels in storage stability has been required, however, an effective measure to respond to these requirements, particularly in spherically-shaped toners, has not yet been found so far.
Further, as a method for improving chargeability of a toner, in particular, a negatively charged toner, it is also known that a fluoride compound is contained in a toner to serve as a charge controlling agent, and the like (Japanese Patent (JP-B) Nos. 2942588, 3102797, and other documents). It is known that when these fluoride resins are used, the fixing ability (fixing temperature range) of the toner degrades, although the chargeability thereof are surely improved, and an effective technique to assure low-temperature fixing property and to prevent a small amount of hot offset events has been desired. There has been an attempt to control the atomic mass of fluoride on the toner surface (Japanese Patent (JP-B) No. 3407521), however, the main purpose of the invention is to improve the chargeability of toner, and the invention does not allow for fixing property, and so the fixing property of the toner degrades undesirably.